The invention relates to adjusting for generating color separations in a multicolor printing machine, changes in the dimensions of printing substrates being taken into account by controlling the image generation.
EP 0 469 282 A2 discloses a method and an apparatus for adjusting generating color separations in a multicolor printing machine. It is used for the mutual coordination of color separations which are fused with the paper, that is to say joined to the paper by melting, separately, in such a way that the next color separation always fits with accurate register onto the color separation or separations already fused. To this end, each printed and fused color separation is measured, and the changes in the dimensions are taken into account for the accurate-register application of the next color separation.
Although control systems are also known which previously register parameters that act on the register and take them into account when controlling the image generation, such as that from U.S. Pat. No. 5,689,757, the teaching there is to take into account the influence of the roughness of the paper on the register when feeding in printing substrates of a different grade. To this end, the roughness of the paper, which is to be printed, is measured. The influence of the paper roughness on the register is then taken into account by the register control before the paper is fed into the printing process. However, a dimensional change in a printing substrate, in particular a paper, because of its anisotropic behavior cannot be taken into account in a corresponding manner.
The invention concerns taking account of changes in the dimensions during printing or after printing as early as before printing when controlling the image generation.
The invention is therefore based on the object of taking into account changes in the dimensions of printing substrates during or after printing as early as during the image generation.
The object is achieved by the fiber orientation of printing substrates being registered and, on the basis of the fiber orientation and values from experience of the behavior of printing substrates depending on the fiber orientation, the control is carried out before the start of printing.
Further, the object is achieved by a sensor for registering the fiber orientation being arranged upstream of the first printing unit, by a store being provided to accommodate values from experience of the behavior of printing substrates depending on the fiber orientation, and by the control device to adjust the devices for producing color separations on the basis of the values from experience before the start of printing and being connected to the sensor and store for the purpose of data transmission.
The invention makes it possible, by associating values from experience with the directions which characterize an anisotropic behavior of a printing material, to take into account correction values as early as during the image generation, with the objective that the prints applied shall have an absolute or relative desired value after a change in the dimensions of the printing material. In this case, the dimensional changes of printing material and print are virtually identical, since there is a difference between these dimensions only in a print-free margin. The invention relates to the processing of all printing materials which have a fiber orientation and therefore exhibit different dimensional changes resulting from different conditions, in particular shrinkage under the influence of ink absorption, absorption of moisture, drying, temperature changes and so on. As a rule, these are printing materials such as paper and paperboard. These generally exhibit a higher shrinkage in the fiber direction than transversely to the latter.
The prior control of the image generation according to the invention is particularly advantageous when multicolor printing machines with digital image generation produce a different printed image during each image generation operation, since in this case proofs are not possible. The same applies to the production of small editions of identical prints.
Developments of the invention provide for the storage and call-up of various values from experience for the control of the image generation. For example, provision can be made for values from experience for various paper grades to be stored, and the relevant values from experience to be taken into account. For this purpose, with respect to the apparatus, provision is made for the store to be loaded with values from experience for various paper grades. The paper grade used can be input with the print job or separately or registered by a sensor.
An alternative configuration provides for at least one paper property to be registered, the values from experience for the at least one paper property to be stored and for the relevant values from experience to be taken into account. In this way, values from experience for various paper thicknesses (calipers) or for various paper weights (grammages) can be provided. The apparatus for implementing the method then provides for at least one sensor to register paper properties, for the store to be loaded with values from experience for various paper properties, and for the control device to be connected to the at least one sensor for the purpose of data transmission. Here, it may be, for example, a sensor for registering the paper thickness or a sensor for registering the paper weight. Of course, it is also possible, however, to input the values from experience for the corresponding paper property with the print job or by hand.
Alternatively or preferably additionally, provision can be made for at least one condition causing the changes in the dimensions of printing substrates to be registered and for values from experience associated with this condition to be taken into account in the control. Examples of such conditions are the atmospheric humidity and the temperature, and further conditions, such as the amount of toner applied, can of course likewise be taken into account. With respect to the apparatus, to this end, provision is made for at least one sensor to register at least one condition that causes the change in the dimensions of printing substrates, for the store to be loaded with values from experience for the at least one condition, and for the control device to be connected to the at least one sensor for the purpose of data transmission. The sensor may be a sensor for registering the atmospheric humidity, a temperature sensor or another sensor. It is possible to take account both of conditions within the printing process and also conditions, which occur after the printing process. This ultimately depends on the objective of the control. Of course, however, values from experience or the conditions, which are associated with stored values from experience, could also be input by hand.
The objectives of the control can be that absolutely predefined dimensions are to be achieved by means of the measure according to the invention, or that relative relationships are to be achieved. For example, provision can be made for the control to be used to achieve predefined dimensions of the prints in the end product. To this end, a control device is such that it takes into account changes in the dimensions in such a way that predefined dimensions can be achieved in the finished prints.
If printing substrates are to be printed on both sides, then the control can also be used to coordinate the dimensions of recto and verso print with one another. The control device is then such that it takes into account the changes in the dimensions in such a way that the dimensions of recto and verso print are coordinated with one another.
However, the invention may also be used in such a way that the control is used to coordinate the dimensions of the prints of the various color separations with one another. The control device is then such that it takes into account the changes in the dimensions in such a way that the dimensions of the prints of the various color separations are coordinated with one another.
As already mentioned, it is also possible for values from experience of conditions which bring about a change in dimensions of printing substrates after printing to be taken into account. Such conditions can be, for example, that the printing substrates heated by the fuser cool down to room temperature, or that they absorb atmospheric humidity or that a drying process takes place. However, the values from experience can also be dimensional changes resulting from further processing, for example as a result of the application of a varnish layer. With respect to the apparatus, provision is then made for the store to be loaded with values from experience which relate to conditions which act on a printing substrate after the printing, and for the control device to be such that it takes into account the values from experience when controlling the image generation in order to achieve predetermined dimensions.